Lung isolation is being used more frequently in both adult and pediatric age groups due to increasing incidence of thoracoscopy and video-assisted thoracoscopic surgery in these patients. Lung isolation and one-lung ventilation (OLV) are indicated in various surgical and nonsurgical procedures. The management of hypoxia resulting with OLV is a stepwise drill of increasing inhaled oxygen, adding positive end-expiratory pressure (PEEP) to ventilated lung, and continuous positive airway pressure or high-frequency jet ventilation (HFJV) to nonventilated side.

Separation of two lungs, termed as “lung isolation,” makes each of them function as an independent unit aiming to provide improved exposure of the surgical field and protection of healthy lung from infected or bleeding one. However, on the flip side of it, OLV causes more airway damage secondary to onsite manipulation. It also leads to significant physiological derangements such as ventilation-perfusion (V/Q) mismatching and early development of hypoxia. Main problem is obligatory shunt through the nonventilated lung. Main compensatory mechanism is hypoxic pulmonary vasoconstriction (HPV).

Development of hypoxemia (arterial oxygen saturation <90%) caused by OLV can be explained by following factors:

Reduction in oxygen stores of the body, due to the disease process and collapse of one lung, the functional residual capacity (FRC), and hence, the oxygen stores of the body get significantly reduced in a situation of OLV [1]

Poor oxygenation as effects of anesthesia and the lateral decubitus position

Compromised ventilation due to compression of ventilated, dependent lung by the weight of mediastinum and by abdominal contents after diaphragmatic paralysis further adds to the gravity of atelectasis of the ventilated lung. Increased closure of small airways with old age, reduced elastic recoil, and the lateral position leads further to more atelectasis [1]

Dissociation of oxygen from hemoglobin due to nonventilation of one out of two lungs causes a reduction in arterial oxygen partial pressures, increase in CO2 levels, and respiratory acidosis. It leads to rapid dissociation of oxygen from hemoglobin (Bohr effect), shown by the steep slope of the oxygen-dissociation curve

HPV is a natural protective reflex that reduces pulmonary blood flow through nonventilated lung by 40%–50% during OLV resulting in moderation of hypoxia. It is triggered at alveolar PO2 <100 mmHg, the degree of which is proportional to the degree of hypoxia below this level. It acts by diverting blood flow away from the underventilated lung, thereby minimizing V/Q mismatch.

It is maximized by maintaining - normocapnia, peak, and plateau airway pressures <30 cm of H2O, restricted use of oxygen (FiO2 <50%). However, controversies exist regarding optimum FiO2 before and during OLV and the limits of acceptable degree of desaturation.[1] These factors apply equally to infants, children, and adults.[2]

For children and adults with unilateral lung disease, V/Q matching is optimal when the patient is placed in the lateral decubitus position with the “healthy” lung in a dependent position. This holds true for both spontaneous and controlled ventilation. Due to gravitational forces, this position results in increased perfusion to the “healthy,” dependent lung and decreased perfusion to the diseased, nondependent lung. As the diseased lung has impaired ventilation at baseline, this preferential distribution of blood flow away from the diseased lung results in optimal V/Q matching.[2]

For VATS performed in the supine position, OLV is essential. However, good surgical exposure can be achieved with single-lumen tube and double-lung ventilation for surgery in posterior mediastinum with the patient in prone position. This is due to the tendency of the structures of anterior mediastinum falling away from the surgical field under the effect of gravity, making a space for the surgeon to work without the ipsilateral lung being collapsed.[1],[2]

Techniques of One-Lung Ventilation in Infants and Small Children (Age <6 years)

The single-lumen endobronchial tubes (EBTs) with smaller bronchial cuffs have a larger “margin of safety” than the uncuffed single-lumen endotracheal (ET) tubes.[2] An important feature of EBTs is a narrow bronchial cuff and a relatively short distance from the proximal age of that cuff to the distal tip of the tube. Thus, there is a lesser chance of bronchial cuff obstructing the right upper lobe bronchus that may occur easily if single-lumen ET tubes (ETTs) are used for this purpose.[3],[4] For emergency situations, normal ETTs can be used, for example, acute contralateral tension pneumothorax and acute airway hemorrhages. Additional methods described in literature for the same purpose include (1) Extraluminal or para-axial placement of external bronchial blockers due to nonavailability of compatible equipment (ET tubes, fiberoptic bronchoscopes etc.) for their coaxial placement.[2] (2) The Fogarty catheter used as a bronchial blocker (Sims Portex Ltd, UK). A size 3 Fogarty catheter can provide effective bronchial blocking for children up to 4 years age. A size 5 Fogarty catheter can be used for most children from 5 to 12 years age.[1],[5] (3) Marraro pediatric ET biluminal tube [Figure 1]. An assembly of two uncuffed tubes joined parallel to each other, with a shorter one as the tracheal part and longer on as the bronchial part. This tube has been reported to be safe and effective in children up to 3 years of age.[1],[4] (4) High-frequency oscillation ventilation (HFOV) and HFJV applied to the nondependent lung, just like CPAP to oxygenate, reduce shunt fraction, and maintain the operated lung in a slightly distended position. Jet ventilation applied through the central lumen can prevent hypoxia during airway exchange.[6]

All double-lumen tubes (DLTs) are curved in two planes. The mainstem which lies in the trachea is concave anteriorly while the more distal bronchial portion is curved at right angle to this with concavity toward the side whose bronchus is to be negotiated (viz., concavity of right-sided DLT bronchial part is toward the right-side). The main argument against the use of right-side DLT is the relative low margin of safety due to this RUL bronchus anatomy. The left-sided tube can serve good for lung isolation of all situations. The right-sided DLTs are required in situ ations where the left main bronchus has been anatomically altered significantly, for example, due to compression by tumor or thoracic aortic aneurysm or where surgical procedure involves the left main bronchus, for example, left main bronchial resection or repair, left pneumonectomy, or lung transplantation.[1]

The gold standard for assessment of positioning of tubes is direct visualization by fiberoptoic bronchoscope.[1],[2] Adequacy of lung isolation is judged clinically by visualization of chest expansion and hearing the breath sounds. Ultrasonography (USG) of the chest can be used as a convenient tool to confirm the adequacy of lung isolation. With the intercostal approach, an interface between the soft tissue of chest wall and aerated lung is seen as a hyperechoic line, “the pleural line.” In ventilated lung, there is a to and fro movement at the pleural line that corresponds to tidal movement of the lung (lung sliding sign). In the nonventilated lung, there is the absence of lung sliding, whereas in collapsed lung, the pleural line moves with a heartbeat in a pulsatile manner (lung-pulse sign). Lung pulse is 93% sensitive and 100% specific for identification of lung collapse. Thus, if lung sliding on one side and lung pulse on other are seen on USG, an adequate “functional lung isolation” can be predicted.[1]

Pulmonary hemorrhage

Hypoxemia due to active pulmonary bleeding is a challenging clinical scenario not usually responsive to maximal support on conventional ventilation. Continuously high mean airway pressure (MAP) produced by HFOV cause tamponade effect on ruptured capillaries with resultant reduction in pulmonary blood flow. The high MAP reduces edema, distends alveoli with resultant improvement in lung recruitment and oxygenation. Furthermore, continuous high pressure will result in fewer pressure swings during the respiratory cycle thus minimize alveolar bleeding. A case series study by Mat Nor et al. demonstrated that in rapid, overwhelming, and potentially fatal form of pulmonary hemorrhage, HFOV can be lifesaving.[7]

Bronchopleural fistula

The primary aim of positive-pressure ventilation in these circumstances is to keep the airway pressure below the critical opening pressure of the fistula. To reduce the flow across a BPF, minimal levels of PEEP, short inspiratory time, low-tidal volumes, and low-respiratory rate are useful. Airway pressures should not exceed 30 cm of H2O.[6],[8]

The incidence of hypoxia during OLV (SpO2 of <90%) is about 5%. This can be prevented by improving preoperative lung function: The standard five-pronged measures for preoperative improvement include reducing irritant exposure including smoking, bronchodilators for airway dilatation, mucolytic agent administration, chest physiotherapy to remove secretions, and antibiotics to treat infection if present.[1]

Steps to Be Taken Sequentially to Evaluate and Treat Hypoxemia during One-Lung Ventilation

Place patient on 100% FiO2

Evaluate position of tube or blocker by auscultation for breath sounds or through fiberoptic bronchoscopy

Apply CPAP (5 cm of H2O) or HFO or HFJV to the nonventilated lung. This is acceptable only in open thoracotomies and not in VATS and thoracoscopic lung resections as this inflation of lung interferes with vision of surgeon

Apply low levels of PEEP (10 cm of H2O) to the dependent lung to improve the FRC

Ensure there is no kinking or obstruction of the tube from secretions

Intermittent lung recruitment maneuvers can be used on operated side of lung

Optimization of Hb levels and cardiac output

Lastly, in case of intractable hypoxia, the surgeon should be informed and asked for reinflation of operative lung or clamping of the pulmonary artery of the nonventilated lung.[1]